The present invention relates to a nickel plated steel sheet or strip featuring excellent corrosion resistance and plating adhesiveness, and a manufacturing method thereof. Specifically, this invention concerns highly corrosion-resistant nickel plated steel sheet or strip suitable for electric/electronic parts, such as battery cells, and stationery, such as ring binders and other materials, and its manufacturing method.
A barrel plating method has been used, but the method has disadvantages of poor productivity and poor uniformity of coating thickness.
In order to solve these problems, a method by which a steel sheet is pre-plated with nickel and pressed later has been replaced by the barrel plating method.
Moreover, in order to improve corrosion resistance of a nickel plated steel sheet, heat-treatment after nickel plating has been proposed by U.S. Pat. No. 2,115,750, for example. The specification of the U.S. patent states only that it is desirable if the thickness of a diffused layer formed by heat-treatment is less than the thickness of nickel plating before heat-diffusion.
TOKUKAI SHO 61 (Japanese Unexamined Patent Publication 1986) 235594 discloses a nickel plated steel sheet with a coating weight of 9-62 g/m.sup.2 per side is heat-treated so as to form a Ni-Fe alloy layer of 4 .mu.m or less.
However, in the above-mentioned U.S. Pat. No. 2,116,750 and TOKU KAI SHO 61-235594, the relation between the whole plating layer and an alloy layer is not clearly described. Especially in relation to plating thickness, the lower limit of the thickness of a Fe-Ni alloy layer is not specified.
In the TOKU KAI SHO 61-235594, the upper limit of diffusion layer thickness is limited to 4 .mu.m or less, as mentioned before. However, for instance, if the plating thickness is 9 g/m.sup.2, and a Fe-Ni alloy layer is 4 .mu.m, the Fe-Ni alloy layer becomes too thick in comparison with the thickness of nickel plating.
Therefore, it is thought that iron in base sheet is diffused and exposed up the surface of the plated layer. If iron is excessively diffused and exposed up to the surface of a plated layer as mentioned before, there is the disadvantage of corrosion resistance degrading.
On the other hand, in case of U.S. Pat. No. 2,115,750, a lower limit of diffusion thickness is not described. Our experimental results showed that when a diffusion thickness is too thin for a plating thickness, sufficient plating adhesiveness and improved corrosion resistance cannot be obtained.
Furthermore, TOKUKAI HEI 3 (Japanese Unexamined Patent Publication) 166388 discloses a method of heat-treatment after nickel plating in which a so-called re-plating method is proposed, whereby a steel sheet is plated with nickel (1.5-9.0 g/m.sup.2) and the nickel plated layer is totally changed to a Fe-Ni alloy layer by heat-treatment, then plated with nickel again.
However, the thickness of the diffusion layer becomes too thick for a plating thickness by this method. In this method, a re-plated layer is hard and non-ductile. Consequently the hard and non-ductile plating layer is to be formed on a Fe-Ni alloy layer, which causes cracks in the plated layer, and moreover, lead to cracks in the Fe-Ni diffusion layer. Due to the cracks, improvement of corrosion resistance is impaired and the re-plated layer does not adhere well to the plating layer. As described above, the conventional technology of heat-treatment after nickel plating does not clearly explain the relation between the thickness of nickel plating and the thickness of a Fe-Ni diffusion layer formed by heat-treatment.
Corrosion resistance is influenced by the quantitative proportion of a Fe-Ni diffusion layer to a thickness of nickel plating. That is, if the thickness of a Fe-Ni diffusion layer is too thick for the plating thickness, corrosion resistance might not be improved, and conversely might be deteriorated when compared with a nickel plated steel sheet or strip not subjected to heat-treatment following nickel plating.
This invention aims to specify the relation between the plating thickness and the thickness of a diffusion layer, and thereby, to provide a nickel plated steel sheet or strip with superior corrosion resistance and plating adhesiveness, and a manufacturing method thereof.